Often off-road trucks are subjected during their routine use to weight loads which differ greatly because of different material density and/or the ability of some material to more tightly pack when loaded into the truck body. As a result, truck bodies which are always filled to their full volume capacity may carry weight loads which exceed the weight capacity of the truck. Repeated occurrences of overloading result in the premature deterioration of the structural integrity of the truck, thus requiring repair or replacement of parts before anticipated. In order to avoid the damage caused by overloading, the truck body can be filled to a volume which assures the truck is not overloaded even for the most dense material. Although underloading may prevent the premature deterioration of the structural integrity of the truck, it sacrifices the truck's load-hauling efficiency. Therefore, an off-road truck which is expensive to operate becomes even more expensive to operate when it is underloaded. Accordingly, there is a need to precisely measure the load carried by an off-road truck. This need has stimulated the development of on-board weighing devices that monitor and measure the truck's load.
Of course, in order to measure the on-board weight of a load carried by a truck, the truck must necessarily incorporate load sensors into its frame and/or body. In a dump-body truck, the body is movable on the truck's frame between lowered and raised positions. To provide for this movement, the body is usually attached to the frame only by a pair of hinge assemblies and a pair of hydraulic cylinders. In one common construction of a dump-body truck, when the truck body is in its lowered position, its entire weight is communicated to the truck frame along a cushioned interface between the truck's frame and body. In this lowered position of the truck body, the hinge assemblies and hydraulic cylinders do not support the weight of the truck body and, therefore, they do not transfer any of the body's weight to the truck frame. By freeing the hinge assemblies and the hydraulic cylinders from the weight of the lowered truck body, the amount of stress on these areas is reduced and, accordingly, their useful life is extended.
Traditionally, in order to provide an on-board weighing device for this type of a dump-body truck, load sensors are incorporated into the hinge assemblies and the hydraulic cylinders. Accordingly, in order to measure the load, the truck body must be lifted from its lowered position by the hydraulic cylinders so that the weight of the load is transferred to the frame through the cylinders and the hinge assemblies. Although the accuracy of the load measurements obtained from load sensors associated with the hydraulic cylinders and the hinge assemblies is satisfactory, the structural integrity of the truck may be degraded by modifications of the hinge assemblies and hydraulic cylinders required to incorporate the load sensors which cause concentration of the load on the frame. Moreover, the impact of falling material onto the bed of the truck is especially severe for the frame of the truck when the body is lifted slightly from its lowered position.
More important than the structural disadvantage of on-board weighing devices which incorporate load sensors in the truck's hinge assemblies and hydraulic cylinders is the disadvantage of requiring the truck's body to be lifted off the frame in order to obtain a weight reading. Because this requirement consumes valuable time otherwise available for loading, hauling, and unloading and because of the concentration of the load on the frame, the truck operator is discouraged from weighing the truck load; it is faster to approximate the load. Since the on-board weighing device interferes with an efficient and smooth hauling operation, there is a tendency to not use the weighing device. Therefore, the advantages of an on-board weighing devices in dump-body trucks have not been fully realized. Also, the requirement of lifting the truck body off the frame in order to obtain a weight measurement prevents continuous or periodic monitoring of the body's weight.
In order to continuously monitor and measure the load carried by a dump-body truck, it is known to use pressure gauges or similar type load sensors in the truck's suspension. Usually, in these types of weighing devices, the fluid pressure within a hydraulic suspension cylinder is sensed. Because of the relatively short stroke of the cylinder and the relatively large amount of frictional resistance to the cylinder's movement (the front cylinders normally also serve as the front axle spindles), the pressure reading is not a satisfactorily accurate indication of the truck's weight. In addition, the modification of the truck's suspension to include load sensors opens the possibility of dangerously degrading the suspension system.